A conventionally known system, which is disclosed, for instance, by Japanese Patent Laid-Open No. 48761/2002, exercises feedback control over a fuel injection amount in accordance with a value detected by an air-fuel ratio sensor that is mounted in an exhaust path of an internal combustion engine. The air-fuel ratio sensor is equipped with a sensor element, which becomes active when heated to an activity temperature, and a heater, which heats the sensor element. This conventional system makes use of a correlation between the sensor element temperature and element impedance to exercise feedback control over the electrical power supply to the heater in order to ensure that the sensor element reaches a predetermined target impedance value. The target impedance is a sensor element impedance that prevails at the activity temperature. When this heater control method is used, it is possible to maintain the sensor element at the activity temperature and steadily keep the air-fuel ratio sensor active.
The above sensor element decreases the element impedance when its temperature increases and increases the element impedance when it deteriorates. Therefore, if the sensor element deteriorates, the element impedance does not decrease to the target impedance when the sensor element reaches its activity temperature. If, in such an instance, heater feedback control is continuously exercised while the target impedance remains unchanged, the sensor element will be heated to a temperature above the activity temperature.
To avoid the above situation, the above conventional system concludes, if the heater is continuously activated for a period longer than predetermined during heater feedback control, that the sensor element is deteriorated, and then increases the target impedance for correction purposes. When this process is performed, it is possible to quickly detect an element impedance increase in the course of sensor element deterioration, increase the target impedance in accordance with sensor element deterioration, and effectively prevent the sensor element from being overheated.
When the above conventional system begins to exercise feedback control over the fuel injection amount by making use of an air-fuel ratio sensor output, it is necessary to judge whether the sensor element is active. This activity judgment can be made, for instance, by monitoring the element impedance after internal combustion engine startup and checking whether the monitored element impedance value is lowered to a predetermined activity judgment value. However, the above-mentioned temperature characteristic is superposed over the element impedance. Therefore, if the activity judgment value is fixed, the same problem arises as in a case where control is exercised until the element impedance coincides with the target impedance. More specifically, the element impedance increases as the sensor element deteriorates; therefore, the activity judgment will be delayed.
Such a delayed activity judgment directly delays the beginning of fuel injection amount feedback control. To obtain an excellent emission characteristic in the internal combustion engine, it is preferred that fuel injection amount feedback control begin as soon as possible. In this respect, the use of the conventional activity judgment method may readily degrade the internal combustion engine's emission characteristic in accordance with air-fuel ratio sensor deterioration.
The delay in activity judgment can be corrected, for instance, by applying the above conventional system's target impedance correction method to the activity judgment value. More specifically, the delay in activity judgment, which is caused by sensor element deterioration, can be avoided by increasing the activity judgment value when sensor element deterioration is detected during an internal combustion engine operation, storing the increased activity judgment value, and using the stored activity judgment value to execute an activity judgment at the next internal combustion engine startup.
However, when the above method is used, before heater feedback control begins (that is, before the sensor element temperature is close to the activity temperature), sensor element deterioration cannot be detected, thereby the influence of deterioration cannot be reflected in the activity judgment. In other words, the activity judgment value correction is constantly delayed by one trip so that the sensor element deterioration cannot be reflected in the activity judgment method in real time at internal combustion engine startup.
Further, to implement the above method, it is necessary to perform a process for correcting the activity judgment value and storing the corrected value, that is, to perform an activity judgment value learning process and exercise complicated control. Moreover, the activity judgment will be delayed until the activity judgment value learning process is completed if the corrected activity judgment value is cleared due to battery replacement or the like according to the above method.
The present invention has been made to solve the above problems. It is an object of the present invention to provide an exhaust gas sensor control device that is capable of determining the degree of sensor element deterioration in real time and executing a prompt activity judgment at all times in an exhaust gas sensor warm-up sequence without resort to activity judgment value learning.